1. Field of the Invention
This invention relates to a workpiece carrier for use in clean environment, such as a clean room.
2. Description of the Related Art
Hitherto, an apparatus disclosed in JP-UM-B-7-28444 has been known as a workpiece carrier for use in a clean room. This workpiece carrier comprises an elongated guide rail, a nut block having ball, end caps, a large number of balls, and a support unit. The elongated guide rail has ball rolling grooves. The nut block has ball rolling grooves respectively provided in both outer faces thereof and opposed to the ball rolling grooves of the guide rail. The end caps are respectively fixed to both axial end portions of the nut block and adapted to communicate each of the ball rolling grooves of the nut block with a ball return passage thereby to form a ball circulating passage. The large number of balls are adapted to circulate along the ball circulating passage. The support unit is fixed to the guide rail and adapted to support a feed screw shaft screwed into the nut block in such a way as to be rotatable and as to be unable to axially move. Further, the nut block has overhanging portions provided in such a way as to be close to and cover the top portions of both side walls of the guide rail. The guide rail is constructed so that an upper portion thereof is covered with an upwardly swelling flexible sealing member by causing the nut block to pass on the guide rail, that a magnetic material is used in the sealing member, and that a rubber magnet for sucking the sealing member is provided on the top portion of the guide rail. Thus, the opening portion is prevented as much as possible from being exposed. Consequently, for example, lubricant is prevented from flying from the inside of the apparatus to the outside thereof.
However, there is a fear that when the nut block moves, abrasive powder is generated in such a workpiece carrier by friction between the end cap, which moves together with this nut block, and the sealing member. In the case of requiring an extremely high degree of cleanness, there is room for further improvement. Thus, JP-A-2000-346165 discloses a workpiece carrier, of which the movable body partly projects to the outside through an opening portion formed by an exterior cover and moves along the opening portion, comprises a pair of elastic sealing members, a first magnetic circuit, and a second magnetic circuit. The elastic sealing members have flexibility and are enabled to cover the opening portion. The first magnetic circuit imparts to the sealing member a suction force acting in a direction in which the opening portion is covered. The second magnetic circuit is provided on the movable body for generating a repulsion force between the first and second magnetic circuits.
In the case of such a workpiece carrier, there is no fear that similarly as in the workpiece carrier described in JP-UM-B-7-28444, abrasive powder is generated by the sliding contact between the movable body and the elastic sealing member.
However, the paired sealing members are mutually agnetized in opposite polarities. Thus, the sealing members intimately contacted with each other should be separated from each other by the passing of the movable body. This requires that a large magnetic force is provided at the side of the movable body, and that a smooth change from the intimately contacted state of the sealing members to the separated state thereof is achieved. Control of a balance among the magnetic forces of these elements and body is necessary for fulfillment of such requirements and difficult to achieve. Thus, the cost thereof is high. Moreover, such a workpiece carrier has a problem in that because a total of four elastic sealing members are needed, this causes an increase in the number of components and a rise in the cost thereof.